1. Field of the Invention
The present invention relates to a method and apparatus for the removal of nitrogen trichloride from chlorine, and more particularly to such a method and apparatus in which waste products detrimental to the environment are avoided.
2. Description of the Prior Art
Chlorine, when produced electrolytically, frequently contains several parts per million of hazardous nitrogen trichloride. This nitrogen trichloride is caused by reaction of ammonia or ammonium ion which is invariably present in the electrolysis brine. Additional ammonia may enter the system from cooling water containing same where direct contact chlorine cooling is employed.
Although the initial concentrations of nitrogen trichloride are rather low, higher concentrations tend to build up due to the great difference in vapor pressures between chlorine (Boiling Point of -34.degree. C) and nitrogen trichloride (Boiling Point of 71.degree. C). Thus even very low concentrations can be hazardous. For example, more than once, one ton chlorine cylinders have been known to explode when empty, and this is believed to have been caused by nitrogen trichloride remaining after chlorine removal.
Thus the hazard is well known, and a number of systems have been proposed to overcome the problem. For example, U.S. Pat. No. 2,705,219 proposes the destruction of nitrogen chloride in the chlorine stream by subjecting the stream to actinic light. Alternatively, U.S. Pat. No. 3,568,409 suggests the destruction of nitrogen trichloride in a chlorine stream (preferably as it leaves the cell room) by contacting the stream with 20% to 30% hydrochloric acid.
Although these patents do provide methods for destroying nitrogen trichloride, they require additional equipment and processing in the work up of the stream of chlorine, and are not in widespread use. In the usual procedures for purifying the chlorine stream coming from the cell room, the stream will reach a cooler or the like where condensate collects. This condensate is mostly liquid chlorine, but this is where the nitrogen trichloride tends to accumulate. One of the common present practices calls for contacting these liquid chlorine bottoms with carbon tetrachloride or chloroform to retain a dilute solution of nitrogen trichloride as the chlorine is allowed to evaporate away. The solvent is then discarded either directly or after addition of caustic.
Due to the fact that chlorinated organic compounds have become recognized as carcinogenic, it is no longer feasible to simply sewer the solvent. However, it is still desirable to use organic solvents to extract the nitrogen trichloride, and the present invention allows for such use.